New ortho-para conversion mechanism in dense solid hydrogen.

Analysis of recent measurements of striking changes in the rate of ortho-para conversion of solid ${\mathrm{H}}_{2}$ up to 58 GPa shows that the conversion mechanism must differ from that at ambient pressure. A new conversion mechanism is identified in which the emerging excitations are coupled to the converting molecules via electric quadrupole-quadrupole rather than nuclear spin-spin interactions. The latter only initiates conversion while the coupling enhancement associated with the new mechanism is ensured by high compression and a gap closing, with the conversion energy diminishing strongly with increasing pressure.